In analog video tape recording and reproducing devices where analog video signals are recorded on parallel tracks extending across a length of a magnetic tape, for example helical tracks, it is known to record the analog audio signal by a stationary transducer along a longitudinal track. During editing, the tape may be moved in either a forward or reverse direction typically by manually rotating the tape reels, or the capstan, or a control knob located on the control panel. During such operation mode, generally referred to as jog mode, the tape speed is dependent on the rotational speed of the knob, while the audio signal is being reproduced from the tape by a stationary playback head. The pitch of the reproduced sound changes with the speed of the relative head-to-tape movement. During the jog mode, reproduction of the audio signal facilitates edit decision making by the operator. The operator listens to the reproduced sound and recognizes portions of the spoken words or music, which enable him to make edit decisions at accurately selected locations on tape. Before making an edit, such as a cut, insert, or a mark on the tape, the operator manually stops the tape at a desired location by any of the above-described means. As it is well known in the art of magnetic recording and playback, when there is no relative transducer-to-tape movement, there is no signal reproduced from the tape. Therefore, when the tape is stopped, the stationary heads do not reproduce any audio signal. The resulting silence is a desirable feature for the editor at stop motion playback.
In digital video tape recorders, as well as in certain types of analog video tape recorders, both the video and audio signals are recorded on helical tracks by one or more rotating heads. One such example are video tape recorders having audio signals recorded as frequency modulated carrier signals. Because of the high rotational speed of the heads relative to the tape speed, the head-to-tape speed is not influenced significantly by changing the longitudinal tape speed. Consequently, the pitch of the reproduced audio signal, which is dependent on that relative speed, remains substantially constant with the changing longitudinal tape speed. When the tape is stopped, for example during editing, the playback head continues to rotate over the same helical track, or groups of tracks, whereby it reproduces over and over again a portion of the audio signal that is recorded on that particular track or tracks. This annoys the editing operator, who has to listen to a relatively loud short fragment of audio signal which is being continuously repeated. Generally it would be possible to obtain silence by manually turning off the audio amplifier, but such operation must be repeated every time after the tape is stopped, and the amplifier has to be turned on again when tape motion is resumed. It is apparent that such an additional repetitive manual operation would significantly slow down the editing process, and therefore it is not useful.
Similarly, during slower than normal tape speed, each recorded track or group of tracks is scanned more than once, and thereby each audio portion is repetitively reproduced, resulting in a similar undesirable audio experience for the editor.
Accordingly, it is an object of the present invention to provide an apparatus and method which would eliminate the undesirable audio signal automatically when the tape is stopped, and reduce the level of that signal when the tape is moving at a selected slow speed, for example below normal tape speed at which the recording was made.
Briefly, in accordance with the method of the present invention, an audio playback signal is reproduced utilizing an audio playback channel. A gain control signal is provided as being dependent on the magnitude of the longitudinal tape speed, and it is utilized to adjust an audio output gain of the audio playback channel as follows.
At zero tape speed the gain control signal reduces the audio output gain to zero. When the tape speed magnitude is changing from zero to a predetermined magnitude, the gain control signal changes the audio output gain in the same direction. When the magnitude of the tape speed exceeds that predetermined magnitude, the gain control signal has a constant value.
In accordance with the preferred method of the invention, a gain control factor is calculated, which has a zero value at zero tape speed, and it increases with an increasing magnitude of the tape speed, up to a predetermined tape speed magnitude. When the magnitude of tape speed exceeds the predetermined magnitude, the gain control factor is equal to a predetermined constant value. The resulting gain control factor is utilized to multiply a preset value of the audio output gain of the audio playback channel, thereby adjusting the audio playback signal level.
The present invention allows playback of audio signals recorded on parallel tracks extending across a length of a magnetic tape, while eliminating the disturbing repetitive reproduction of the audio signals at zero tape speed, and reducing the volume of such disturbing signals within a predetermined low tape speed range.